A principal objective in paper manufacturing techniques is to achieve properties of paper which are as uniform as possible. In the drying stage of paper manufacture, the paper web shrinks more in the lateral areas than in the middle areas. This may even have the consequence that the paper in the lateral areas has a deteriorating effect on the other parts of the paper web. In view of the desired quality of the paper, it would be preferable that the shrinkage of paper is small and uniform.
In prior art methods, attempts have been made to avoid the above problem by increasing the negative pressure in the suction rolls arranged in the drying section. When the negative pressure is increased, the web shrinks less in the middle area, but the difference between the lateral areas and the middle area may even become larger. Thus, the result obtained by such a solution is not desirable.
One solution for the problem has been suggested in Finnish Patent Application No. 895928. In the solution provided therein, a profile of negative pressure that reduces the problem is applied just to a part of the path of the paper in the drying section. In addition, in Finnish Patent Application No. 861291, a lateral band solution is suggested, which provides for the prevention and equalization of the shrinkage through the entire path of the paper. Drawbacks of this method include wear of the lateral bands, how to make the bands remain in their grooves, and unusability of the lateral strip.